Drain Water Heat Recovery – We’ve ordered one!

Three years ago, I wrote about my discovery of drain water heat recovery (also known as grey water heat recovery). Simply put, they appear to offer the same energy savings as a solar hot water system, but for a tiny fraction of the price. I read up on it quite a bit. They are well supported overseas, and there are plenty on the market internationally. So when the opportunity arose to purchase one, I looked at the two locally available examples, and picked the one that was able to be installed vertically, the EnergyDrain. Being locally made, and cheaper also helped in the decision.

Figure 1: Figure EnergyDrain

Despite some semi-effortful attempts, in my background reading, I hadn’t come across any criticism. So I went ahead and ordered one. However, semi-fatefully, a week or so after placing the order, I mentioned this systems in a forum discussing solar water heating, and David Haywood (wearing his engineering hat) said:

I’ve done heaps of modelling on these systems and they are good in theory. The problem is the HX cost and the cleaning. Most systems use some sort of horrible draino-type stuff every few weeks.

And went on to be explicitly critical of the horizontally installed ones. The more expensive GFX that I ruled out, seems much less prone to fouling, but that is due to its vertical installation, which isn’t possible for our house.

The main problem is that the building up of scum inside the heat exchanger will reduce its efficiency; though I’m not sure if it will ever meant that efficiency reduces to zero. Anyway, we’ve bought one, it’s arriving any day now, so I think it’s worth quantifying whether we will see much in the way of savings, and whether scum build-up is an issue.

Fortuitously, once the system has been installed, we will switch our hot water cylinder onto night rate, which is separately metered, so our energy consumption for hot water will be easily measured.

In order to accurately estimate the savings, I plan to have the plumber install a bypass loop round the heat exchanger, as well as a Y-regular joint, with an inspection opening, as illustrated below

To answer the first question, what are the real-world savings, if any, I’ll use an ABBA design, as illustrated below. I’ll route the cold water through the bypass for a week, then two consecutive weeks with the heat exchanger (HXC) in operation, followed by a week with the bypass back on. The advantage of the ABBA design is that if there are any other variables changing over time, affecting our hot water energy use (eg, changes in the weather, the system getting dirtier), this should cancel it out. Efficiency can then be estimated as the ratio of energy decrease during B divided by energy used during A (here (15-10)/15 = 33%).

Then, to assess the influence of dirtiness, I can plot energy use over time, and then occasionally clean it out. My faked data below would suggest that most energy savings is lost after 90 days (dotted lines indicate cleaning, and here the efficiency declines, so kW used increases), so would suggest cleaning rather more frequently. Obviously, I secretly hope for data that does not look like this, as it would imply that I should clean it every month or two.

Obviously, I’d be much happier if it looked something like this, with the efficiency declining a bit, then plateauing.

Welcome any feedback on my design, and otherwise, I guess there will be an update in a month or two.

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Roy Morgan predicting National landslide?

UPDATE: This post was written early yesterday (Monday), but I chickened out of posting it, because I thought it was too much of a ballsy call. However, Roy Morgan are themselves making the call, so I think it is worthy of discussion. In particular, because this is not a recent change, but part of a consistent pattern for them.

The Herald Digipoll, ONE News Colmar Brunton, Fairfax-Research International, and 3 News Reid Research are all showing support for National dropping, but Roy Morgan has National support steady, and comfortably above 50%.

Polls are invariably reported with their Margin of Error, an estimate of the precision for a party with exactly 50% support. The Margin of Error is not very useful for comparing whether there is a difference between two parties, and it certainly is no use at all for considering change over time. And change over time ought to be what we are interested in. Is support for Party X climbing or dropping?

One crude way of doing this is to add a line fitted through the datapoints, but one of the hidden aspects of variation, as I tried to illustrate on Friday is that different polls will use slightly different methods, which may mean that that poll produces consistently different results in a certain direction. As I noted then, Roy Morgan generally shows lower support for National. However, over the last month, the other polling companies have all shown a clear drop, while Roy Morgan have had National’s support climbing.

Roy Morgan sample over a week to a fortnight, which could explain them being slower to catch on to this most recent trend. Alternatively, it may be that some particular element of their survey method is producing a consistently different result for them. This is brave: if they are proved right on election night, it could be that their method is superior. If they are wrong, then they may want to reconsider how they are polling. Irrespective of the outcome, however, it brings me back to a recent summary of Daniel Kahneman’s work looking at the success of fund managers: whoever is most successful in a given year (or election?) may just be due to chance.

UPDATE 2: The downward trend exists for the other 3 polls, but because Fairfax Research International and Herald Digipoll have relatively few data points, it is hard to display in a tidy fashion. Also, Roy Morgan’s trend does not diverge in the same way for Labour and the Greens, only National.

The data is derived from wikipedia, and fit with a modified version of the graph there, using a LOESS fit, with a span value of 0.3.

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Reading the Political Tea Leaves

Despite mentioning tea in the title, this post is about opinion polls, not the ACT of drinking tea.

There are a number of sites that maintain graphs of New Zealand political polls, notably Rob Salmond at Pundit and, curiously, Wikipedia. Wikipedia post their data in tabular form, and provide the code for R underlying the creation of their graphs, allowing anyone with certain degree of geek cred to have a hack (ie, me). Yesterday, someone on wikipedia requested an updated version focussing only on more recent polls, which I had a go at, including making the fit line a bit more sensitive to change.

Changing the sensitivity of the fit line, makes it seem like there more movement than there has been in a while, and this graph was subsequently featured on the DimPost and then the Listener.

The next question was about whether the different polling parties differentially contribute to such a rolling poll. Their sample sizes are pretty uniform, at around 800. Roy Morgan contribute over half (68 of 121) with their poll regularly conducted over several days. 3 News Reid Research have fewer polls (16) but all conducted on a single day.

But how do the different parties fare in the polls (in order of polling).

Firstly, you can really get a sense from this graphic how much more regular Roy Morgan are, and their estimates are pretty consistently low, relative to the other pollsters. 3 News Reid Research is fairly consistently high. However, in the latest few polls, they have had some lower numbers for National.

3 News Reid Research is consistently lower on Labour, which combined with the above, suggests they favour National relative to the other pollsters. The Herald Digipoll has higher values for Labour most of the time.

The Herald Digipoll and ONE News Colmar Brunton are consistently lower for the Greens than other pollsters. However, in the most recnet polls, Herald Digipoll and ONE News Colmar Brunton both show much higher numbers than usual for the Greens.


  • There is definitely variation attributable to the pollsters.
  • It does seem like there is some change, at least with the polls. Whether this translates to anything meaningful for next Saturday, who knows. I plan to add ACT and NZ First later for completeness.
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Warmth from the sun

The other week, Cr Fliss Butcher suggested that there should be a ban on south-facing homes in Dunedin. Predictably (as can be seen in the comments thread in the link), this has been met with a hail of derision, but also some support. Personally, I think New Zealand building standards seem to be always out of date. Houses that were built as little as 10 years ago seem embarrassingly bad by current standards because of this. It was only in 2008 that double glazing became sort of mandatory, and many houses from the 90s (and some even up until 2008) have embarrassingly little insulation.

There are already rules on south, east and west facing glazing; if a house has more than 30% of its glazing on these walls, thermal modelling is required. This seems a more nuanced approach than simply banning south-facing homes. However, as always it seems that the rules could be more aggressive. I think that if you can afford to build a new house, then you can afford to spend a little more money to make sure that you are building a real asset. In essence, if you are going to build a new house in Dunedin, you may as well build a bloody warm one.

Research in Dunedin by economist Dr Paul Thorsnes backs this up. Houses built after 1978 (when insulation first became compulsory), command a hefty price premium relative to pre-insulation era houses. Similarly, houses that receive more sun in mid-winter are worth more than houses that receive less sun. This effect is particularly profound for pre-insulation era houses; a 3.9% increase for each additional hour of mid-winter sun. Perhaps unsurprisingly, Thorsnes lives in a sun-trap property.

But this is also a tale of how far we have come. When Austrian refugee architect Ernst Plischke designed a house in Christchurch c.1940, it was initially rejected because it did not have any* south facing windows. The issue was, of course, that the street was to the south, and under the rules of the time, the house had to address the street. The solution was eventually to add a few windows.

Fast forward 50-60 years, and the idea of an L-shaped house, opening to the sun and an outdoor living space is now taken as an almost given. The design seems surprisingly contemporary, as do some of Plischke’s other designs, such as the house he designed for Bill Sutch in Wellington, below.

*I’m pretty sure the design actually had none, and that he added some small windows, but I’m having to rely on my memory of this exhibition: ERNST PLISCHKE – ARCHITECT, City Gallery, Wellington, 5 September-28 November 2004.

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Shower Water Heat Recovery, Green Stars, and loan schemes

Several new University of Otago buildings have been constructed to 5 Star Green Building ratings: rainwater harvesting, grey water for flushing toilets, and lots of other energy efficiency features. This seems very laudable, except I’ve been extremely dubious about the merit of the shower water heat exchangers in the Hunter Centre.

The idea is that the warm water going down the drain pre-heats the water going to the shower, so less energy is used to heat that water for the same warmth of shower. This seems like a fantastic and practical idea, and according to the propaganda of the people that sell them, can easily save 25-40% of hot water heating costs (GFX, Vaportec). Except who showers in a building full of tutorial rooms? In fact, the demand has been so overwhelming, that the showerheads in the Hunter Centre have been removed.

Equally laudably, the Dunedin City Council is planning to provide loans to install solar hot water heating. Dunedin isn’t the best place in the country for solar hot water (although it *does* work). It would also seem sensible to provide loans for hot water heat pumps, as in Dunedin they are at least as effective as solar hot water. This is in part because during winter, solar hot water needs an electrical ‘top up’.

In contrast to solar hot water ($5000-$10,000) or hot water heat pumps ($5000), a heat exchanger for the grey water from the shower is closer to $700. They can be easily retrofitted to many existing houses (you merely need underfloor access; they are more efficient installed vertically, but can be installed horizontally), but can also be used in conjunction with solar or heat pump systems — and would effectively reduce the size of system that needs to be installed. They are also entirely compatible with gas hot water systems.

Simple, easy to install, and comparatively cheap. I would challenge the Dunedin City Council to not just provide loans for their installation, but also to see if they can get a discount for a bulk order. I’m ready to forgive the Hunter Centre: Even if no-one has ever had a shower there, it may be that getting the idea out there is what matters.

UPDATE: I’ve had a trawl through the published literature. There is not a great deal out there, but this paper argues that drainwater heat recovery is more cost effective than solar hot water heating, and there is sufficient evidence to show that these systems work.

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Why cash and buses don’t mix

One of the things that causes public transport to fail is when it starts to run late as passengers take too long to load.

The perfect opposite of this is the Hong Kong MTR (underground). All platforms have screen doors (so you can’t fall on the tracks, or be pushed). This also means you know where the train door is going to be. Now, in the image below, you can see some arrows on the platform. Basically, the white arrows encourage waiting passengers to wait at the sides of the doors, and green arrow indicates that exiting passengers go straight through the middle. Being Hong Kong, this works pretty well, and so there is pulse of people exiting, who aren’t hindered by those getting on, and vice versa. This means the train doesn’t ‘dwell’ in the station very long, meaning the route is quicker, and the chance of delay is less.

Underground systems also neatly illustrate why cash is the kryptonite of public transport. The transaction for your ride doesn’t occur on the platform, so it doesn’t hinder loading and unloading. It usually happens in a big entrance hall with a row of shiny gates (not so shiny in some systems), where people who have tickets already sail through at speed. Those who need to buy tickets are off to the side, and those who need to use a person to accomplish their sales transaction are usually marginalised further still. Thus, the tourist, or the infrequent user doesn’t hold up anybody else.

With trams, you usually buy your ticket from a machine on the street (e.g., Zurich, St Etienne, Geneva) or from a machine inside the tram in Lisbon (but not from the driver).

With buses on the other hand, cash fares almost always involve the driver, and the more cash fares there are, the longer the wait. What then, are some solutions for dealing with cash on buses.

Cardiff — don’t give change. Saves time, really screws you over if the smallest thing you have is a 20 pound note (so you won’t do it again).

Los Angeles — don’t give change. Saves time, but the US obsession with paper money means that you need to try to feed one of their super-ratty $1 bills into a note feeder, which can tak aaaaaggggges.

Umeå, Sweden — don’t take cash AT ALL. You either have a their bus pass card, or you have to use a credit card (no pin or signature required, just a swipe).

Dijon — use a round fare amount (1 euro), so that people don’t have to fiddle with extra coins. Travel cards accepted at all doors on the bus for faster loading.

Lisbon — make the electronic fares so cheap that even most tourists would have one. RFID card costs 50c. Fare with card 80c. Fare without card 1.40. Integrated ticketing for funiculars, trams, buses, and metro really make getting a card simple.

Now, let’s compare and contrast with Dunedin:

  • Cash accepted on bus (bonus points for time wasted with drivers sometimes grumbling over breaking of small notes) and fares requiring complicated change
  • RFID cards stupidly expensive ($5), compared to Lisbon card (50c, made of light card, easily replaceable)
  • RFID Go Card only gives measly discount (therefore, casual users unlikely to bother with high upfront cost and little discount)

The practical upshot of this is that if the buses are busier, it inevitably means each stop takes increasingly long as it is slow boarding people, and a busier bus becomes a victim of its own suggest.

Solutions for Dunedin:

  1. Get cheaper RFID cards (Lisbon’s cardboard ones work just fine).
  2. Create a real incentive to get a Go Card. Perhaps make a casual fare $3.00, or you can get a Go Card on the spot for $5, with $4.50 credit. This way, if you plan to take two bus trips, buying a Go Card is a no brainer.
  3. Make all the casual fares whole dollar values (simplifies change giving).

Longer term solutions:

  1. With the tap-and-go credit cards coming to NZ, enable these for use (and make them cheaper than cas).
  2. Get stronger RFID system, so that you don’t have to take your card out of your wallet/bag/purse
  3. Consider altering the zoning system, so that people with Go Card don’t need to interact with driver. In Lisbon, this is achieved by having no zoning. In Wellington, you tap your card as you get on AND as you exit, which then calculates zones travelled (if you don’t tap your card on exit, you get charged to the end of the route).


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Why primary school children shouldn’t do town planning.

It seems that fairly large tracts of Florida make for some amazing art, but it’s not town planning. More like doodling with patterns, and being, ‘Oh, that could be be a town layout’.

Some of them are beautiful. Some of them remind me of towns I’ve built with SimCity*

Really do click through. In addition to some of them being amazingly beautiful as a satellite image, and amazingly stupid as town planning fails, there are links to google maps and streetview, so you can explore these neighbourhoods, including the under-developed and overgrown ones.

*The difference between these grids and those that you might make in SimCity is if there is one thing that SimCity has taught me is the importance of good rail connections. Building roads just leads to congestion – quel surprise! Reinforced of course, by visiting places that actually have amazing public transport (Hi Lisbon! Hi Toulouse!).

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